The current density in an integrated circuit device becomes larger as the device becomes smaller. High current densities can cause electromigration, which in turn may result in device failure. Electromigration refers to the transport of mass in metals under the influence of current. It occurs by the transfer of momentum from electrons to positive metal ions. When a high current passes through thin metal conductors, e.g, in an IC, metal ions accumulate in some regions and voids form in other regions. The accumulation of metal ions may result in a short circuit to adjacent conductors, while the voids may result in an open circuit.
The electromigration resistance of some types of conductors, e.g., aluminum film conductors, may be increased by using several techniques, including alloying with copper, e.g., Al with 0.5% Cu, encapsulating the conductor in a dielectric, or incorporating oxygen during film deposition. Those and other techniques, however, are both expensive and unreliable in comparison to avoiding the source of the problem, i.e., preventing high current densities by specifying minimum conductor sizes in accordance with the expected operating conditions of the device. There are a variety of techniques in use in the electronics industry for specifying the minimum size of the conductors of an IC. However, none of the known techniques is easy to use; in fact, most require either tedious manual calculations or highly specialized software.
Accordingly, a primary goal of the present invention is to provide an automatic and reliable tool for verifying that a circuit layout complies with such electromigration specifications. A further goal of the present invention is to provide a tool that employs existing circuit analysis software, thus making it inexpensive and reliable. These goals are achieved by the present invention, a specific embodiment of which is described in this specification.